Washington (AFP) – In a spectacular twist to the US probe into allegations of Russian election meddling, Donald Trump’s former top advisor Michael Flynn pleaded guilty Friday to lying over his contacts with Moscow, and promised to cooperate with the wide-ranging investigation.

While Russia continues to deny it interfered in the 2016 election, US intelligence agencies say they have established beyond doubt that it did so, with the aim of tilting the outcome towards the Republican Trump.

But a question at the heart of the FBI’s investigation — whether Trump’s campaign colluded with Russian meddling — remains far from answered.

Here is what we know about the investigation:

– How it all began –

In October 2016 — one month before the election — US intelligence agencies publicly blamed Russia for hacking and leaking embarrassing documents from the Democratic Party during the presidential campaign.

Weeks before leaving office, on December 29, president Barack Obama announced sanctions and the expulsion of 35 Russian diplomats in retaliation.

In January, US intelligence chiefs went on to publish a report concluding that President Vladimir Putin masterminded the hacking and disinformation campaign with the aim of damaging Trump’s rival Clinton.

– The investigations –

The Justice Department, the FBI and intelligence agencies all launched investigations into Russia’s alleged interference in the campaign. In Congress, three Senate committees and one House committee also opened overlapping investigations into the Russia controversy.

On May 9, Trump sacked the head of the FBI, James Comey, in an apparent bid to hamper the Russia probe. The sacking backfired, leading to the Justice Department’s appointing of a more powerful, independent counsel, Robert Mueller, who now heads the federal investigation.

– The first charges –

On October 30, Mueller announced his first three indictments.

Paul Manafort, Trump’s one-time campaign chairman, and Manafort’s deputy Rick Gates, were arrested on money laundering and tax-related charges in relation to their work for Moscow-backed Ukrainian leader Viktor Yanukovych.

A third man, George Papadopoulos, a foreign policy advisor to the campaign who sought to arrange a meeting between Trump and Putin, pleaded guilty to lying, and agreed to cooperate with the probe.

Court filings make clear that top campaign officials were aware of Papadopoulos’s communications with Russians, including an offer of damaging information on Trump’s rival Clinton. That is seen as the sharpest evidence yet of possible collusion.

– Flynn falls… again –

Exactly one month later Mueller struck again, filing charges against Flynn, a retired three star general who advised Trump on national security during the campaign, and served the new president in the White House for 23 days before being fired over his Russian contacts.

On Friday Flynn pleaded guilty to lying to investigators over those contacts with Russian ambassador Sergey Kislyak, which took place in December 2016.

Flynn, court filings showed, worked with Kislyak to enlist Russia’s help in blocking a UN Security Council resolution critical of Israel. And he urged Kislyak to dissuade Moscow from retaliating to the sanctions announced by Obama in December.

Tantalizingly, court documents related to his plea deal stated that he was directed by “very senior” transition team officials on the contacts. That has sparked a guessing game in Washington as to who those officials are.

– What is Mueller up to? –

Mueller, who has a large team of seasoned prosecutors skilled in tracing money and flipping witnesses, is methodically picking off people around Trump who can be easily prosecuted and forced to cooperate.

Flynn and Manafort were always expected to be hit early and hard. They worked closely with Trump and would have large amounts of information about the campaign roles of others. Analysts speculate that Gates, not nearly as well-off as the multi-millionaire Manafort, could be more willing to supply evidence against his former bosses.

Papadopoulos was a minor player but officially was under the guidance of Jeff Sessions, who now serves as Trump’s attorney general and who is accused of being less than forthcoming about his own Russian contacts. Mueller would need to tread very carefully in investigating Sessions, who heads the Justice Department, though he has recused himself from the Russia probe.

– So was there collusion? –

The US president — whose refusal to strongly condemn Russian interference has fuelled lingering suspicions — dismisses all talk of collusion with Russia as “fake news” and a “witchhunt.”

And while the indictments to date indicate there were more interactions between Team Trump and Moscow than first admitted, nothing has emerged that directly ties Trump to any meddling.

“Nothing about the guilty plea or the charge implicates anyone other than Mr. Flynn,” White House lawyer Ty Cobb said Friday, adding that he was confident Mueller’s probe was nearing “a prompt and reasonable conclusion.”

Yet even without evidence of collusion, the court filings raise questions about the conduct of Trump’s campaign.

Papadopoulos last year developed multiple contacts in London with people he understood were close to the Kremlin, including one whom he described as “Putin’s niece.” He talked openly in emails to senior campaign officials about a possible meeting with Russia’s ambassador in England and about obtaining dirt the Russians held on Clinton.